First evidence of $CP$ violation in beauty baryon to charmonium decays

Abstract

A study of the difference in the $CP$ asymmetries between ${\Lambda}^0_b \rightarrow J / \psi p \pi^-$ and ${\Lambda}^0_b \rightarrow J / \psi p K^-$ decays, $\Delta {\cal A}_{CP}$, is performed using proton-proton collision data collected by the LHCb experiment in the years 2015--2018, corresponding to an integrated luminosity of $6 \,{\rm fb}^{-1}$. This quantity is measured to be $ \Delta {\cal A}_{CP}=(4.03\pm 1.18\pm 0.23)\%$, where the first uncertainty is statistical and the second is systematic. When combined with the previous LHCb result, a value of $\Delta {\cal A}_{CP} = (4.31 \pm 1.06 \pm 0.28)\%$ is obtained, corresponding to a significance of $3.9\sigma$ against the $CP$ symmetry hypothesis. Studies of triple-product asymmetries, which provide an additional probe of $CP$ violation, show no significant deviation from $CP$ symmetry.